Depends on where you want the socket. Most caravans are wired as radial circuits, so the easiest way is simply to find the nearest 240V socket or junction box and extend from that - it's difficult to overload the wiring as the circuit breaker is only 10 amps. You could also cut a wire and insert a junction box if there's sufficient slack in the cable. Don't take it from the fridge circuit, however, as that will be wired to the 5 amp circuit breaker.